1. Field of the Invention
The present invention relates to a thermal recording material, more precisely to a thermal recording material having good lightfastness and in particular to a thermal recording material having the advantages of good lightfastness and image fixation.
2. Description of the Related Art
Relatively inexpensive and capable of being processed in simple and reliable recording appliances not requiring maintenance, thermal recording materials for forming an image upon being provided with a heat by a thermal head, and the like are widely used in the art. Heretofore, there has been a high demand for high-performance thermal recording materials,for example, with improved high image quality or storage stability and the like. Thus, research relating to color density, image quality, storability, and the like has been performed.
Various types of thermal recording materials have been heretofore known, including, for example, those in which electron-donating leuco-dyes and electron-accepting compounds in which they are reacted to form colors, and those in which diazo compounds such as diazonium salts and couplers in which they are reacted to form colors.
The recent development in the art toward full-color thermal recording materials is noticeable. In general, full-color thermal recording materials have a laminate structure of yellow, magentaand cyan-coloring layers, in which the respective layers are thermally colored to form full-color images.
One example of such full-color thermal recording materials has a laminate structure of color-forming layers (thermal recording layers) comprising an electron-donating leuco-dye and an electron-accepting compound and color-forming layers comprising a diazo compound and a coupler, in which yellow, magenta and cyan color-forming layers are laminated in that order from its top.
One method of improving the storability and the image stability against light and heat of thermal recording materials is to encapsulate the electron-donating leuco-dyes and the diazo compounds in microcapsules. According to this method, the electron-donating leuco-dyes and the diazo compounds in thermal recording materials are isolated from water and bases that may promote their decomposition, and, as a result, the shelf life of the thermal recording materials is prolonged (Tomomasa Usami, et al., xe2x80x9cThe Journal of the Electrophotography Society of Japanxe2x80x9d, Vol. 26, No. 2, 1987, pp. 115-125).
In heat-responsive microcapsules, a type of microcapsules in which the glass transition point of the capsule wall is slightly higher than room temperature, the capsule wall is substance-impervious at room temperature but is substance-pervious at a temperature not lower than its glass transition point. Accordingly, when electron-donating leuco-dyes or diazo compounds are encapsulated with this capsule wall and electron-accepting compounds, couplers or bases are outside the microcapsules in the thermal recording material, then the encapsulated electron-donating leuco-dyes or diazo compounds can be kept stable for a long time and a color image can be readily formed by heating the material. In addition, the color image thus formed on the material can be readily fixed thereon through exposure to light (photofixation).
One general method of encapsulating a core substance of electron-donating dye precursors or diazo compounds into microcapsules comprises dissolving the core substance in a hydrophobic solvent (oily phase), followed by emulsifying and dispersing this mixture in an aqueous solution having a water-soluble polymer dissolved therein (aqueous phase) using a homogenizer or the like, with a monomer or a prepolymer, which will be the microcapsule wall, being added to one or both of the oily phase and the aqueous phase, to thereby polymerize the monomer or the prepolymer in the interface between the oily phase and the aqueous phase or precipitate the polymer to form a wall of the polymer compound. The method is described in detail, for example, in xe2x80x9cMicrocapsulesxe2x80x9d (by Tomoji Kondo, Nikkan Kogyo Shinbun Publishing, 1970); and xe2x80x9cMicrocapsulesxe2x80x9d (by Tamotsu Kondo, et al., Sankyo Publishing, 1977).
However, the above method of microcapsule formation is problematic in that the aqueous solution used forms a lot of bubbles. The bubbles often enter exhaust dusts and therefore lower the production efficiency, and also lower the properties such as the stability of the photographic materials that contain the microcapsules formed according to the method. Therefore, a surfactant that serves as a defoaming agent is generally added to the system of forming microcapsules according to the method. One typical example of the surfactant is sodium dodecylbenzenesulfonate. However, this is still unsatisfactory for completely preventing the generation of bubbles in forming microcapsules when the conditions of this method.
The sensitivity to heat of the microcapsules mentioned above is one important factor thereof. Therefore, if the sensitivity to heat of the microcapsules in one layer of a thermal photographic material is not stable, the quality of the photographic material decreases.
Moreover, when a full color thermal recording material, which is formed by stacking multiple thermal recording layers, uses highly sensitive microcapsules, a problem in that the thermal recording layers comprising microcapsules may generate colors by heat, which is added to cause a color forming reaction in the upper layers. This can cause a mixing of colors and an inferior thermal differation. Thus, when thermal recording layers are superimposed, it is preferable to use microcapsules having low heat sensitivity in the lower layers by the support.
For controlling the sensitivity of microcapsules, the reaction temperature in forming the microcapsules is particularly important. In addition, for satisfactorily preventing the generation of bubbles in microcapsule formation, the surfactant alone is unsatisfactory. Therefore, the reaction condition including the reaction temperature in microcapsule formation must be suitably controlled. However, it is difficult to stably form microcapsules of low sensitivity under the reaction conditions which prevent bubbles from being formed, and it has heretofore been difficult to produce full-color thermal recording materials of good thermal differentiation.
The present invention is to solve the problems in the prior art noted above, and its object is to provide a thermal recording material having the advantages of superior production suitability, production stability and thermal differentiation.
The means of the invention for solving the problems as above are mentioned below.
Specifically, the invention provides a thermal recording material comprising a support and at least one thermal recording layer disposed on the support, wherein the layer comprises at least one electron-accepting compound, and microcapsules, which encapsulate an electron-donating dye precurser and are formed using a compound represented by the following general formula (I) and a compound represented by the following general formula (II): 
wherein each of R1 and R2 represents an alkyl group; n is an integer selected from 0 to 3; and M represents Na, Mg, K or Ca.
The invention also provides a thermal recording material comprising a support and at least one thermal recording layer disposed on the support, wherein the layer comprises at least microcapsules, which encapsulate a diazo compound, and a coupler, for reacting with the diazo compound to produce a color, which microcapsules are formed using a compound represented by the following general formula (I) and a compound represented by the following general formula (II): 
wherein each of R1 and R2 represents an alkyl group; n is an integer selected from 0 to 3; and M represents Na, Mg, K or Ca.
Moreover, the present invention provides the thermal recording material comprising a support and at least one thermal recording layer disposed on the support, the layer including at least an electron-accepting compound, microcapsules, which encapsulate an electron-donating dye precurser, microcapsules, which encapsulate a diazo compound, and a coupler, for reacting with the diazo compound to produce a color.
In the thermal recording material of the invention, the color density of the microcapsules at a recording energy of 100 mJ/mm2 is at most 0.20.
The thermal recording material of the invention is described in detail hereinunder.
Thermal Recording Material:
A thermal recording material of the present invention comprises a support and at least one thermal recording layer disposed on the support, the layer including at least an electron-accepting compound, and microcapsules, which encapsulate an electron-donating dye precursor, and are formed using a compound represented by the following general formula (I) (hereinafter, this may be referred to as a xe2x80x9csurfactant (I)xe2x80x9d) and a compound represented by the following general formula (II) (hereinafter, this may be referred to as a xe2x80x9csurfactant (II)xe2x80x9d).
In another aspect, the thermal recording material of the invention comprises, on a support, at least microcapsules, which encapsulate a diazo compound, and a coupler, for reacting with the diazo compound to produce a color, which microcapsules are formed using a compound represented by the following general formula (I) and a compound represented by the following general formula (II): 
wherein R1 represents an alkyl group, and n is from 0 to 3. 
wherein each of R1 and R2 represents an alkyl group; n is an integer selected from 0 to 3; and M represents Na, Mg, K or Ca.
When forming the microcapsules that encapsulate the electron-donating dye precursor or the diazo compound (both of these will be referred to hereinafter as a xe2x80x9ccolor-forming componentxe2x80x9d) to be in the thermal recording material of the invention, the surfactants (I) and (II) can be used together to satisfactorily prevent a generation of bubbles in the microcapsule formation. Accordingly, the thermal recording material of the invention has excellent production suitability and performance, such as stability.
To stably form microcapsules of low sensitivity, some amount of heating is necessary. In the invention, it is possible to prevent bubbles from being formed in the microcapsule formation even at high temperatures since the surfactants (I) and (II) are used together therein, and thus microcapsules of low sensitivity can be stably formed.
The thermal recording material of the invention includes at least a thermal recording layer on a support, and the thermal recording layer may have a single-layered or multi-layered structure. If desired, the material may have other layers, such as light-transmittance controlling layer and a protective layer in accordance with necessity.
Thermal-Recording Layer:
The thermal recording layer in the invention includes microcapsules, which encapsulate in a color-forming component and are formed using the surfactants (I) and (II).
Surfactants:
The surfactant (I) is a compound of the following general formula (I): 
wherein R1 represents an alkyl group. The alkyl group preferably has from 4 to 12 carbon atoms, and more preferably from 6 to 10 carbon atoms.
Specifically, examples of the alkyl group include t-octyl groups, n-octyl groups, n-hexyl groups, t-hexyl groups and n-dodecyl groups. Of those, t-octyl groups and n-octyl groups are preferable, and n-octyl groups are more preferable.
In formula (I), n is an integer selected from 0 to 3. In general, the surfactant (I) for use in the invention is amixture of compounds of formula (I) where n is an integer selected from 0 to 3, however, the range of n is not specifically defined.
An example of the surfactant (I) is mentioned below. However, the present invention is not limited to this example. 
The surf actant (II) for use in the invention is a compound of the following general formula (II): 
wherein R2 represents an alkyl group. The alkyl group preferably has from 8 to 16 carbon atoms, more preferably from 10 to 14 carbon atoms. The alkyl group may be substituted.
Specifically, examples of the alkyl group includes n-octyl groups, t-octyl groups and n-dodecyl groups. Of those, n-dodecyl groups and t-dodecyl groups are preferable, and n-dodecyl groups are more preferable.
In formula (II), M represents Na, Ca, Mg or K, and is preferably Na.
Examples of the surfactant (II) are mentioned below. However, the invention is not limited to these examples. 
Method of Microcapsule Formation:
In the invention, at least an electron-donating dye precursor or a diazo compound is encapsulated in microcapsules.
For encapsulating the color-forming component in microcapsules, any known method may be used. For example, one preferable method of microcapsule formation is interfacial polymerization, which is performed as follows: An electron-donating dye precursor (or diazonium salt compound), which is a color-forming component, is dissolved or dispersed in a slightly water-soluble or water-insoluble organic solvent to prepare an oily phase, this is mixed with an aqueous phase having a water-soluble polymer dissolved therein, and the surfactants (I) and (II) are added to the resulting mixture, which is then emulsified and dispersed by a homogenizer or the like, and heated to cause interfacial polymerizing reaction, which forms a microcapsule wall of the polymer substance at the surface of oil drops. According to this method, microcapsules having a uniform size can be formed within a short period of time, and the recording material containing them has good storability.
In the method of microcapsule formation, the reaction temperature is preferably from 20 to 70xc2x0 C. and, more preferably from 40 to 70xc2x0 C. To stably form microcapsules of low sensitivity, the reaction temperature is preferably falls from 60 to 70xc2x0 C. and, more preferably from 65 to 70xc2x0 C. If the surfactants (I) and (II) are combined, themethodhas sufficient anti-foamability, even at a high reaction temperature of from 65 to 70xc2x0 C., and microcapsules of low sensitivity can be stably formed. Here, xe2x80x9cmicrocapsules of low sensitivityxe2x80x9d indicates microcapsules that require high energy (high temperature) to form a color, or in other words, microcapsules, which do not form a color until they are heated to a sufficiently high temperature.
In the present invention, the surfactants are generally dissolved in water. The molar ratio of the surfactant (I) [x] to the surfactant (II) [y], x/y is preferably from 8/2 to 2/8, more preferably from 6/2 to 2/6, and is even more preferably 2/1. When the molar ratio falls is from 8/2 to 2/8, the surfactant mixture prevents bubbles from being formed, from accumulating excessively in the reaction tank, and can perform a sufficient defoaming effect.
The surfactants (I) and (II) may be combined with any other known surfactant, as long asthis does not interfere with the effect of the present invention. Examples of the known surfactant include sodium methylnaphthalenesulfonates and 2-ethylhexyl sulfosuccinates.
When the surfactants (I) and (II) are combined with any other known surfactant, the surfactants (I) and (II) preferably comprise at least 50% by weight, more preferably at least 80% by weight of the total amount of the surfactants used.
Examples of the organic solvent include low-boiling-point auxiliary solvents such as acetates, methylene chlorides and cyclohexanones, and/or phosphates, phthalates, acrylates, methacrylates, other carboxylates, aliphatic acid amides, alkylated biphenyls, alkylated terphenyls, alkylated naphthalenes, diarylethanes, chloroparaffins, alcohol solvents, phenol solvents, ether solvents, mono-olefin solvents, and epoxy solvents.
Examples of the high-boiling-point organic solvents are tricresyl phosphate, trioctyl phosphate, octyldiphenyl phosphate, tricyclohexyl phosphate, dibutyl phthalate, dioctylphthalate, dilaurylphthalate, dicyclohexylphthalate, butyl esters of olefinic acids, diethylene glycol benzoate, dioctyl sebacate, dibutyl sebacate, dioctyl adipate, trioctyl trimellitate, acetyltriethyl citrate, octyl maleate, dibutyl maleate, isoamylbiphenyl, chloroparaffins, diisopropylnaphthalene, 1,1xe2x80x2-ditolylethane, monoisopropylbiphenyl, diisopropylbiphenyl, 2,4-di-tert-amylphenol, N,N-dibutyl-2-butoxy-5-tert-octylaniline, 2-ethylhexyl hydroxybenzoate, polyethylene glycol.
Of those, alcohol solvents, phosphate solvents, carboxylate solvents, alkylated biphenyls, alkylated terphenyls, alkylated naphthalenes, and diarylethanes are especially preferable.
In addition, carbonization inhibitors such as hindered phenols and hindered amines may be added to the high-boiling-point solvents. The high-boiling-point solvents preferably have unsaturated fatty acids, for example, xcex1-methylstyrene dimers. A commercial product of a-methylstyrene dimers is, for example, MSD100 (trade name, producted by Mitsui Toatsu Chemical Co. Ltd.).
An example of the water-soluble polymer is polyvinyl alcohol, for example, polyvinyl alcohol, silanol-modified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, amino-modified polyvinyl alcohol, itaconic acid-modified polyvinyl alcohol, styrene-maleic anhydride copolymer, butadiene-maleic anhydride copolymer, ethylene-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, polyacrylamide, polystyrenesulfonic acid, polyvinylpyrrolidone, ethylene-acrylic acid copolymer, and gelatin. Among these, carboxy-modified polyvinyl alcohol is preferable.
The water-soluble polymer may be combined with a hydrophobic polymer emulsion or latex. Examples of the emulsions latexes include styrene-butadiene copolymers, carboxy-modified styrene-butadiene copolymers and acrylonitrile-butadiene copolymers.
Examples of the polymer substance, which forms the microcapsule walls include polyurethane resin, polyurea resin, polyamide resin, polyester resin, polycarbonate resin, aminoaldehyde resin, melamine resin, polystyrene resin, styrene-acrylate copolymer resin, styrene-methacrylate copolymer resin, gelatin, and polyvinyl alcohol. Among these examples, polyurethane-polyurea resins are particularly preferable.
For example, when a polyurethane-polyurea resin is used for the microcapsule wall material, microcapsule walls are produced as follows: a microcapsule wall precursor such as a polyisocyanate is mixed with an oily medium (oily phase), which will be a core substance to be encapsulated. Furthermore, a second substance (e.g., polyols, polyamines) that reacts with the microcapsule wall precursor to form a capsule wall is mixed in an aqueous solution of a water-soluble polymer (aqueous phase). The oily phase is emulsified and dispersed in the aqueous phase and then heated to cause interfacial polymerization to form a microcapsule wall around every oil drop.
Examples of the polyisocyanate compound are given below, however, the invention is not limited to these examples: diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 2,6-tolylene diisocyanate, 2,4-tolylene diisocyanate, naphthalene-1,4-diisocyanate, diphenylmethane-4,4xe2x80x2-diisocyanate, 3,3xe2x80x2-diphenylmethane-4,4xe2x80x2-diisocyanate, xylene-1,4-diisocyanate, 4,4xe2x80x2-diphenylpropane diisocyanate, trimethylene diisocyanate, hexamethylene diisocyanate, propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, cyclohexylene-1,2-diisocyanate, cyclohexylene-1,4-diisocyanate; triisocyanates such as 4,4xe2x80x2,4xe2x80x3-trihenylmethane triisocyanate, toluene-2,4,6-triisocyanate; tetraisocyanates such as 4,4xe2x80x2-dimethylphenylmethane-2,2xe2x80x2,5,5xe2x80x2-tetraisocyanate; and isocyanate prepolymers such as hexamethylene diisocyanate-trimethylolpropane adduct, 2,4-tolylene diisocyanate-trimethylolpropane adduct, xylylene diisocyanate-trimethylolpropane adduct, tolylene diisocyanate-hexanetriol adduct.
In accordance with necessity, two or more of these examples may be used in combination. Among those examples, those having at least three isocyanate groups in the molecule are particularly preferable.
In the method of microcapsule formation, the organic solvent in which the microcapsule wall precursor and the second substance, with which it reacts, as well as other components such as couplers (and electron-accepting compound), organic bases and sensitizers are dissolved have the same meaning as the organic solvent mentioned hereinabove.
The size of the microcapsules is preferably from 0.1 to 2.0 xcexcm, and more preferably from 0.2 to 1.5 xcexcm.
Electron-Donating Dye Precursor:
Examples of the electron-donating dye precursor include triarylmethane compounds, diphenylmethane compounds, thiazine compounds, xanthene compounds, and spiropyran compounds. Of those, triarylmethane compounds and xanthene compounds are preferable, as they form color images of high density.
Specifically, examples of the compounds for the dye precursor include 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (crystal violet lactone), 3,3-bis(p-dimethylamino)phthalide, 3-(p-dimethylaminophenyl)-3-(1,3-dimethylindol-3-yl)-phthalide, 3-(p-dimethylaminophenyl)-3-(2-methylindol-3-yl)phthalide, 3-(o-methyl-p-diethylaminophenyl)-3-(2-methylindol-3-yl)-phthalide, 4,4xe2x80x2-bis(dimethylamino)benzhydrin benzyl ether, N-halophenyl leucoauramine, N-2,4,5-trichlorophenyl leucoauramine, rhodamine-B-anilinolactam, rhodamine(p-nitroanilino)lactam, rhodamine-B-(p-chloroanilino)lactam, 2-benzylamino-6-diehtylaminofluorane, 2-anilino-6-diethylaminofluorane, 2-anilino-3-methyl-6-diethylaminofluorane, 3-anilino-3-methyl-6-cyclohexyldethylaminofluorane, 2-anilino-3-methyl-6-isoamylethylaminofluorane, 2-(o-chloroanilino)-6-diethylaminofluorane, 2-octylamino-6-diethylaminofluorane, 2-ethoxyethylamino-3-chloro-3-diethylaminofluorane, 2-anilino-3-chloro-6-diethylaminofluorane, benzoyl leucomethylene blue, p-nitrobenzyl leucomethylene blue, 3-methylspyrodinaphthopyran, 3-ethylspyrodinaphthopyran, 3,3xe2x80x2-dichlorospyrodinaphthopyran, 3-benzylspyrodinaphthopyran, 3-propylspyrodibenzopyran.
The coating amount of the electron-donating dye precursor is preferably from 0.1 to 1 g/m2 in the thermal recording layer, for the same reason as that for the diazonium salt compound mentioned above.
Electron-Accepting Compound:
Examples of the electron-accepting compound include phenol derivatives, salicylic acid derivatives, and hydroxybenzoates. Among these examples, bisphenols and hydroxybenzoates are particularly preferable. Specifically, they include the following:
2,2-Bis(p-hydroxyphenyl)propane (bisphenol A), 4,4xe2x80x2-(p-phenylenediisopropylidene)diphenol (bisphenol P), 2,2-bis(p-hydroxyphenyl)pentane, 2,2-bis(p-hydroxyphenyl)ethane, 2,2-bis(p-hydroxyphenyl)butane, 2,2-bis(4xe2x80x2hydroxy-3xe2x80x2,5xe2x80x2-dichlorophenyl)propane, 1,1-(p-hydroxyphenyl)cyclohexane, 1,1-(p-hydroxyphenyl)propane, 1,1-(p-hydroxyphenyl)pentane, 1,1-(p-hydroxyphenyl)-2-ethylhexane, 3,5-di(xcex1-methylbenzyl) salicylic acid and polyvalent metal salts thereof, 3,5-di(tert-butyl)salicylic acid and its polyvalent metal salts, 3-xcex1,xcex1-dimethylbenzylsalicylic acid and polyvalent metal salts thereof, butyl p-hydroxybenzoate, benzyl p-hydroxybenzoate, 2-ethylhexyl p-hydroxybenzoate, p-phenylphenol, and p-cumylphenol.
The electron-accepting compound content of the thermal recoding layer is preferably from 0.1 to 30 parts by weight relative to 1 part by weight of the electron-donating dye precursor.
Diazo Compound:
The diazo compound to be encapsulated into microcapsules includes, for example, diazonium salt compounds of the following formula (1):
xe2x80x83Arxe2x80x94N2+Xxe2x88x92xe2x80x83xe2x80x83(1)
wherein Ar represents an aromatic group, and Xxe2x88x92 represents an acid anion.
The diazonium salt couples with a coupler, which will be described below, to form a color when heated, or decomposes when exposed to light. Depending on the site and the type of the substituent in the Ar moiety therein, the maximum absorption wavelength of the diazonium salt compound can be controlled.
Examples of the diazonium group to form the salts are 4-(p-tolylthio)-2,5-dibutoxybenzenediazonium, 4-(4-chlorophenylthio)-2,5-dibutoxybenzenediazonium, 4-(N,N-dimethylamino)benzenediazonium, 4-(N,N-diethylamino)benzenediazonium, 4-(N,N-dipropylamino)benzenediazonium, 4-(N-methyl-N-benzylamino)benzenediazonium, 4-(N,N-dibenzylamino)benzenediazonium, 4-(N-ethyl-N-hydroxyethylamino)benzenediazonium, 4-(N,N-diethylamino)-3-methoxybenzenediazonium, 4-(N,N-dimethylamino)-2-methoxybenzenediazonium, 4-(N-benzoylamino)-2,5-diethoxybenzenediazonium, 4-morpholino-2,5-dibutoxybenzenediazonium, 4-anilinobenzenediazonium, 4-[N-(4-methoxybenzoyl)amino]-2,5-diethoxybenzenediazonium, 4-pyrrolidino-3-ethylbenzenediazonium, 4-[N-(1-methyl-2-(4-methoxyphenoxy)ethyl)-N-hexylamino]-2-hexyloxybenzenediazonium, 4-[N-(2-(4-methoxyphenoxy)ethyl)-N-hexylamino]-2-hexyloxybenzenediazonium, 2-(1-ethylpropyloxy)-4-[di-(di-n-butylaminocarbonylmethyl)amino]benzenediazonium, 2-benzylsulfonyl-4-[N-methyl-N-(2-octanoyloxyethyl)]-aminobenzenediazonium.
The maximum absorption wavelength xcexmax of the diazonium salt compound is preferably no more than 450 nm, and more preferably from 290 to 440 nm. If xcexmax of the compound is longer than 450 nm, the storability of the thermal recording material containing the compound will be poor; but if shorter than the range mentioned above, the image fixation and the image storage stability of the recording material will be reduced and the color of the images formed on the material will inferior, depending on the combination of the compound with a coupler to be combined with it. The coupler will be described hereinunder.
The diazonium salt compound preferably has at least 12 carbon atoms, a solubility in water is at most 1% and in ethyl acetate of at least 5%.
These diazonium salt compounds may be used either singly or in combinations of two or more, depending on the object of the compounds for color control.
Of the diazonium salts compounds mentioned above, those of the following structural formulae (1) to (3) are more preferable in view of their effect to form dyes of better color hue and their effect to ensure better image fixation and image storage stability. 
In the structural formula (1), Ar represents a substituted or unsubstituted aryl group.
Examples of the substituents for the aryl group include alkyl groups, alkoxy groups, alkylthio groups, aryl groups, aryloxy groups, arylthio groups, acyl groups, alkoxycarbonyl groups, carbamoyl groups, carbamido groups, sulfonyl groups, sulfamoyl groups, sulfonamido groups, ureido groups, halogen atoms, amino groups, and heterocyclic groups. These substituents may be further substituted.
The aryl group for Ar preferably has from 6 to 30 carbon atoms, for example, groups of phenyl, 2-methylphenyl, 2-chlorophenyl, 2-methoxyphenyl, 2-butoxyphenyl, 2-(2-ethylhexyloxy)phenyl, 2-octyloxyphenyl, 3-(2,4-di-t-pentylphenoxyethoxy)phenyl, 4-chlorophenyl, 2,5-dichlorophenyl, 2,4,6-trimethylphenyl, 3-chlorophenyl, 3-methylphenyl, 3-methoxyphenyl, 3-butoxyphenyl, 3-cyanophenyl, 3-(2-ethylhexyloxy)phenyl, 3,4-dichlorophenyl, 3,5-dichlorophenyl, 3,4-dimethoxyphenyl, 3-(dibutylaminocarbonylmethoxy)phenyl, 4-cyanophenyl, 4-methylphenyl, 4-methoxyphenyl, 4-butoxyphenyl, 4-(2-ethylhexyloxy)phenyl, 4-benzylphenyl, 4-aminosulfonylphenyl, 4-N,N-dibutylaminosulfonylphenyl, 4-ethoxycabonylphenyl, 4-(2-ethylhexylcarbonyl)phenyl, 4-fluorophenyl, 3-acetylphenyl, 2-acetylaminophenyl, 4-(4-chlorophenylthio)phenyl, 4-(4-methylphenyl)thio-2,5-butoxyphenyl, and 4-(N-benzyl-N-methylamino)-2-dodecyloxycarbonylphenyl. However, the invention is not limited to these groups.
These groups may be further substituted with any of alkyloxy groups, alkylthio group, substituted phenyl groups, cyano groups, substituted amino groups, halogen atoms, and heterocyclic groups.
In the structural formula (1), R21 and R22 each independently represent a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group. R21 and R22 may be the same or different.
In case where the group for R21 and R22 is substituted, examples of the substituent for the group include an alkoxy group, an alkoxycarbonyl group, an alkylsulfonyl group, a substituted amino group, a substituted amido group, an aryl group, and an aryloxy group. However, the invention is not limited to those examples.
The alkyl group for R21 and R22 preferably has from 1 to 18 carbon atoms, examples include, groups of methyl, trifluoromethyl, ethyl, propyl, isopropyl, butyl, sec-butyl, t-butyl, pentyl, isopentyl, cyclopentyl, hexyl, cyclohexyl, octyl, t-octyl, 2-ethylhexyl, nonyl, octadecyl, benzyl, 4-methoxybenzyl, triphenylmethyl, ethoxycarbonylmethyl, butoxycarbonylmethyl, 2-ethylhexyloxycarbonylmethyl, 2xe2x80x2,4xe2x80x2-diisopentylphenyloxymethyl, 2xe2x80x2,4xe2x80x2-di-t-butylphenyloxymethyl, dibenzylaminocarbonylmethyl, 2,4-di-t-amylphenyloxypropyl, ethoxycarbonylpropyl, 1-(2xe2x80x2,4xe2x80x2-di-t-amylphenyloxy)propyl, acetylaminoethyl, 2-(N,N-dimethylamino)ethyl, 2-(N,N-diethylamino)propyl, methanesulfonylaminopropyl, acetylaminoethyl, 2-(N,N-dimethylamino) ethyl, and 2-(N,N-diethylamino)propyl.
The aryl group for R21 and R22 preferably has from 6 to 30 carbon atoms, examples include, groups of phenyl, 2-methylphenyl, 2-chlorophenyl, 2-methoxyphenyl, 2-butoxyphenyl, 2-(2-ethylhexyloxy)phenyl, 2-octyloxyphenyl, 3-(2,4-di-t-pentylphenoxyethoxy)phenyl, 4-chlorophenyl, 2,5-dichlorophenyl, 2,4,6-trimethylphenyl, 3-chlorophenyl, 3-methylphenyl, 3-methoxyphenyl, 3-butoxyphenyl, 3-cyanophenyl, 3-(2-ehtylhexyloxy)phenyl, 3,4-dichlorophenyl, 3,5-dichlorophenyl, 3,4-dimethoxyphenyl, 3-(dibutylaminocarbonylmethoxy)phenyl, 4-cyanophenyl, 4-mehtylphenyl, 4-methoxyphenyl, 4-butoxyphenyl, 4-(2-ethylhexyloxy)phenyl, 4-benzylphenyl, 4-aminosulfonylphenyl, 4-N,N-dibutylaminosulfonylphenyl, 4-ethoxycarbonylphenyl, 4-(2-ethylhexylcarbonyl)phenyl, 4-fluorophenyl, 3-acetylphenyl, 2-acetylaminophenyl, 4-(4-chlorophenylthio)phenyl, 4-(4-methylphenyl)thio-2,5-butoxyphenyl, and 4-(N-benzyl-N-methylamino)-2-dodecyloxycarbonylphenyl. However, the invention is not limited to those examples.
These groups may be further substituted with alkyloxy groups, alkylthio groups, substituted phenyl groups, cyano groups, substituted amino groups, halogen atoms, and heterocyclic groups.
In the structural formula (2), each of R24, R25 and R26 each independently represents a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group, and R24, R25 and R26 may be the same or different.
Examples of the substituent for the substituted groups include an alkyl group, an alkoxy group, an alkylthio group, an aryl group, an aryloxy group, an arylthio group, an acyl group, an alkoxycarbonyl group, a carbamoyl group, a carbamido group, a sulfonyl group, a sulfamoyl group, a sulfonamido group, an ureido group, a halogen atom, an amino group, and a heterocyclic group.
The alkyl group for R24, R25 and R26 preferably has from 1 to 18 carbon atoms, including, for example, the alkyl groups mentioned hereinabove for R21 and R22 in formula (1), and 1-methyl-2-(4-methoxyphenoxy)ethyl, di-n-butylaminocarbonylmethyl and di-n-octylaminocarbonylmethyl groups.
The aryl group for R24, R25 and R26 has the same meaning as that for R21 and R22 in formula (1). However, the invention is not limited to these groups.
The groups may be further substituted with alkyloxy groups, alkylthio groups, substituted phenyl groups, cyano groups, substituted amino groups, halogen atoms, and heterocyclic groups.
In the structural formula (2), Y represents a hydrogen atom, or OR23; and R23 represents a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group.
Examples of the substituent for the substituted groups include an alkyl group, an alkoxy group, an alkylthio group, an aryl group, an aryloxy group, an arylthio group, an acyl group, an alkoxycarbonyl group, a carbamoyl group, a carbonamido group, a sulfonyl group, a sulfamoyl group, a sulfonamido group, an ureido group, ahalogen atom, an amino group, and a heterocyclic group.
Y is preferably a hydrogen atom or an alkyloxy group OR23 in which R23 is an alkyl group, in view of color control of the compound.
The alkyl group for R23 has the same meaning as that for R21 and R22 in formula (1), however, the invention is not limited to this.
The aryl group for R23 has the same meaning as that for R21 and R22 in formula (1), however, the invention is not limited to this. The aryl group may be substituted with any of alkyloxy groups, alkylthio groups, substituted phenyl groups, cyano groups, substituted amino groups, halogen atoms, and heterocyclic groups.
In the structural formula (3), each of R27 and R28 independently represents a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group, and R27 and R28 may be the same or different.
Examples of the substituent for the substituted groups include an alkyl group, an alkoxy group, an alkylthio group, an aryl group, an aryloxy group, an arylthio group, an acyl group, an alkoxycarbonyl group, a carbamoyl group, a carbamido group, a sulfonyl group, a sulfamoyl group, a sulfonamido group, an ureido group, a halogen atom, an amino group, and a heterocyclic group.
The alkyl group for R27 and R28 has the same meaning as that for R21 and R22 in formula (1). However, the invention is not limited to this.
The aryl group for R27 and R28 has the same meaning as that for R21 and R22 in formula (1). However, the invention is not limited to this. The aryl group may be substituted with any of alkyloxy groups, alkylthio groups, substituted phenyl groups, cyano groups, substituted amino groups, halogen atoms, and heterocyclic groups.
In the structural formulae (1) to (3), Xxe2x88x92 represents an acid anion. Examples of the acid for the anion include polyfluoroalkylcarboxylic acids having from 1 to 9 carbon atoms, polyfluoroalkylsulfonic acids having from 1 to 9 carbon atoms, boron tetrafluoride, tetraphenylboron, hexafluorophosphoric acid, aromatic carboxylic acids, and aromatic sulfonic acids. Of those, preferred are hexafluorophosphoric acids in view of the crystallinity of the compounds.
Examples of the diazonium salt compounds of the structural formulae (1) to (3) are given below. However, the invention is not limited to these formulae. 
The diazonium salt compounds of the structural formulae (1) to (3) may be used herein either singly or in combinations of two or more. Further, the diazonium salt compounds of the structural formulae (1) to (3) may be combined with any other known diazonium salt compounds, depending on the objects such as color control and the like.
The coating amount of the diazonium salt compound is preferably from 0.05 to 2 g/m2, and more preferably from 0.1 to 1 g/m2 in the thermal recording layer. If the content of the compound in the layer is smaller than 0.05 g/m2, the layer might not be able to obtain sufficient color density; but if larger than 2 g/m2, the coatability of the coating liquid for the layer may become inferior.
Coupler:
The coupler that couples with the diazonium salt compound mentioned above to form a dye may be any and every one capable of coupling with the diazonium salt compound to form a dye in a basic and/or neutral atmosphere.
All 4-equivalent couplers that are used in silver halide photographic materials are usable in the invention, and may be selected for use herein depending on the object of the invention including the intended color of the images to be formed.
For example, active methylene compounds having a methylene group adjacent to the carbonyl group thereof, phenol derivatives and naphthol derivatives are usable for the couplers in the invention.
Among them, compounds of the following formula (2) and their tautomers are especially preferred for use herein.
E1xe2x80x94CH2-E2xe2x80x83xe2x80x83(2) 
wherein each of E1 and E2 independently represents an electron-attracting group, and may be the same or different.
The electron-attracting group is a substituent having a positive Hammett""s value xcex4. Preferred examples thereof are acyl group including groups of acetyl, propionyl, pivaloyl, chloroacetyl, trichloroacetyl, trifluoroacetyl, 1-methylcyclopropylcarbonyl, 1-ethylcyclopropylcarbonyl, 1-benzylcyclopropylcarbonyl, benzoyl and 4-methoxybenzoyl, thenoyl groups; an alkoxycarbonyl group including groups of methoxycarbonyl, ethoxycarbonyl, 2-methoxyethoxycarbonyl and 4-methoxyphenoxycarbonyl groups; a carbamoyl group including carbamoyl, N,N-dimethylcarbamoyl, N,N-diethylcarbamoyl, N-phenylcarbamoyl, N-[2,4-bis(pentyloxy)phenyl]carbamoyl, N-[2,4-bis(octyloxy)phenyl]carbamoyl and morpholinocarbonyl groups; an alkylsulfonyl or arylsulfonyl group including groups of methanesulfonyl, benzenesulfonyl and toluenesulfonyl groups; a phosphono group such as a diethylphosphono group; a heterocyclic group including benzoxazol-2-yl, benzothiazol-2-yl, 3,4-dihydroquinazolin-4-on-2-yl and 3,4-dihydroquinazolin-4-sulfon-2-yl groups; a nitro group, an imino group, and a cyano group.
E1 and E2 may be bonded to each other to form a ring. The ring to be formed by E1 and E2 is preferably a 5-membered or 6-membered, carbon or hetero ring.
Examples of the couplers are resorcinol, phloroglucine, 2,3-dihydroxynaphthalene, 2,3-dihydroxynaphthalene-6-sodium sulfonate, 1-hydroxy-2-naphthoemorpholinopropylamide, 2-hydroxy-3-naphthalenesodium sulfonate, 2-hydroxy-3-naphthalenesulfonanilide, 2-hydroxy-3-naphthalenesulfonmorpholinopropylamide, 2-hydroxy-3-naphthalenesulfon-2-ethylhexyloxypropylamide, 2-hydroxy-3-naphthalenesulfon-2-ethylhexylamide, 5-acetamido-1-naphthol, 1-hydroxy-8-acetamidonaphthalene-3,6-sodium disulfonate, 1-hydroxy-8-acetamidonaphthalene-3,6-disulfodianilide, 1,5-dihydoxynaphthalene, 2-hydroxy-3-naphthoemorpholinopropylamide, 2-hydroxy-3-naphthoeoctylamide, 2-hydroxy-3-naphthoeanilide, 5,5-dimethyl-1,3-cyclohexadione, 1,3-cyclopentanedione, 5-(2-n-tetradecyloxyphenyl)-1,3-cyclohexanedione, 5-phenyl-4-methoxycarbonyl-1,3-cyclohexanedione, 5-(2,5-di-n-octyloxyphenyl)-1,3-cyclohexanedione, N,Nxe2x80x2-dicyclohexylbarbituric acid, N,Nxe2x80x2-di-n-dodecylbarbituric acid, N-n-octyl-Nxe2x80x2-n-octadecylbarbituric acid, N-phenyl-Nxe2x80x2-(2,5-di-n-octyloxyphenyl)barbituric acid, N,Nxe2x80x2-bis(octadecyloxycarbonylmethyl)barbituric acid, 1-phenyl-3-mehtyl-5-pyrazolone, 1-(2,4,6-trichlorophenyl)-3-anilino-5-pyrazolone, 1-(2,4,6-trichlorophenyl)-3-benzamido-5-pyrazolone, 6-hydroxy-4-methyl-3-cyano-1-(2-ethylhexyl)-2-pyridone, 2,4-bis(benzoylacetamido)toluene, 1,3-bis(pivaloylacetamidomethyl)benzene, benzoylacetonitrile, thenoylacetonitrile, acetacetanilide, benzoylacetanilide, pivaloylacetanilide, 2-chloro-5-(N-n-butylsulfamoyl)-1-pivaloylacetamidobenzene, 1-(2-ethylhexyloxypropyl)-3-cyano-4-methyl-6-hydroxy-1,2-dihydropyridin-2-one, 1-(dodecyloxypropyl)-3-acetyl-4-methyl-6-hydroxy-1,2-dihydropyridin-2-one, and 1-(4-n-octyloxyphenyl)-3-tert-butyl-5-aminopyrazole.
The details of the couplers are described in JP-A Nos.4-201483, 7-223367, 7-223368, and7-323660, and in Japanese Patent Application Nos. 5-278608, 5-297024, 6-18669, 6-18670, 7-316280, 8-027095, 8-027096, 8-030799, 8-12610, 8-132394, 8-358755, 8-358756, and 9-069990.
Examples of the couplers of formula (2) are mentioned below, however, the invention is not limited to these examples. 
The coupler content of the thermal recording layer in the invention is preferably from 0.1 to 30 parts by weight relative to 1 part by weight of the diazonium salt compound in the layer.
Other Components:
Organic Base
An organic base is preferably added to the photosensitive thermal recording layer in the invention to promote the coupling reaction of the diazonium salt with the coupler in the layer.
The organic base is preferably in the layer along with the diazonium salt and the coupler, and the organic base may be in the layer singly or in combinations of two or more.
Examples of the organic base include nitrogen-containing compounds such as tertiary amines, piperidines, piperazines, amidines, formamidines, pyridines, guanidinesandmorpholines. In addition, organic bases described in JP-B 52-46806, JP-A Nos. 62-70082, 57-169745, 60-94381, 57-123086, 58-1347901, and 60-49991, JP-B Nos. 2-24916, and 2-28479, JP-A Nos. 60-165288, and 57-185430 are also usable herein.
Among these examples, piperazines such as N,Nxe2x80x2-bis(3-phenoxy-2-hydroxypropyl)piperazine, N,Nxe2x80x2-bis[3-(p-methylphenoxy)-2-hydroxypropyl]piperazine, N,Nxe2x80x2-bis[3-(p-methoxyphenoxy)-2-hydroxypropyl]piperazine, N,Nxe2x80x2-bis(3-phenylthio-2-hydoxypropyl)piperazine, N,Nxe2x80x2-bis[3-(xcex2-naphthoxy)-2-hydroxypropyl]piperazine, N-3-(xcex2-naphthoxy)-2-hydroxypropyl-Nxe2x80x2-methylpiperazine, 1,4-bis{[3-(N-methylpiperazino)-2-hydroxy]propyloxy}-benzene; morpholines such as N-[3-(xcex2naphthoxy)-2-hydroxy]propylmorpholine, 1,4-bis(3-morpholino-2-hydroxy-propyloxy)benzene, 1,3-bis(3-morpholino-2-hydroxy-propyloxy)benzene; piperidines such as N-(3-phenoxy-2-hydroxypropyl)piperidine, N-dodecylpiperidine; and guanidines such as triphenylguanidine, tricyclohexylguanidine, dicyclohexylphenylguanidine are particularly preferable.
When the organic base is included in the thermal recording layer, an amount thereof is preferably from 0.1 to 30 parts by weight relative to 1 part by weight of the diazonium salt compound therein.
Sensitizer
In addition to the organic base mentioned above, a sensitizer may be added to the thermal recording layer to promoting the color formation in the layer.
The sensitizer acts to increase the color density of the color images formed by heat, or to lower the minimum color-forming temperature of the thermal recording layer. Specifically, it lowers the melting point of the coupler, the organic base and the diazonium salt in the layer, or the softening point of the capsule wall of the microcapsules in the layer, thereby enhancing the reactivity of the diazonium salt, the organic base and the coupler.
Specifically, low-melting-point organic compounds having an aromatic group and a polar group to a suitable degree in the molecule are preferable as the sensitizer. Examples include benzyl p-benzyloxybenzoates, xcex1-naphthyl benzyl ethers, xcex2-naphthyl benzyl ethers, phenyl xcex2-naphthoates, phenyl xcex1-hydroxy-xcex2-naphthoates, xcex2-naphthol (p-chlorobenzyl) ethers, 1,4-butanediol phenyl ethers, 1,4-butanediol p-methylphenyl ethers, 1,4-butanediol p-ethylphenyl ethers, 1,4-butanediol m-methylphenyl ethers, 1-phenoxy-2-(p-tolyloxy)ethanes, 1-phenoxy-2-(p-ethylphenoxy)ethanes, 1-phenoxy-2-(p-chlorophenoxy)ethanes, and p-benzylbiphenyls.
Binder
The binder to be in the thermal recording layer may be any known water-soluble polymer compound or latex.
Examples of the water-soluble polymer compound include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, starch derivatives, casein, arabic gum, gelatin, ethylene-maleic anhydride copolymer, styrene-maleic anhydride copolymer, polyvinyl alcohol, epichlorohydrin-modified polyamide, isobutylene-maleinsalicylic anhydride copolymer, polyacrylic acid, polyacrylamide, and their modified derivatives; and the latex includes, for example, styrene-butadiene rubber latex, methyl acrylate-butadiene rubber latex, and polyvinyl acetate emulsion.
Antioxidant
It is also preferable to add known antioxidants such as those mentioned below to the thermal recording layer for improving the fastness to light and heat of the color images formed and for reducing the yellowness by light of the non-printed area (non-image area) of the recording material after image fixation thereon.
Examples of the antioxidants that are usable herein are described in EP-A Nos. 223739, 309401, 309402, 310551, 310552, and 459416, GP-A No. 3435443, JP-A Nos. 54-48535, 62-262047, 63-113536, 63-163351, 2-262654, 2-71262, 3-121449, 5-61166, and 5-119449, and USP Nos. 4,814,262, and 4,980,275.
In the present invention, the mode of using the couplers, the organic bases, the sensitizers and other components is not particularly limited. For example, these components may be solid (1) dispersed; (2) emulsified and dispersed; (3) polymer dispersed; (4) latex dispersed, or (5) encapsulated into microcapsules.
Constitution of Thermal Recording Material:
Concrete embodiments of the constitution of full-color thermal recording materials of the invention are described below.
The thermal recording material of the invention is a monochromatic thermal recording material having, on a support, one thermal recording layer specifically defined herein, or a full-color thermal recording material having a laminate layer structure of multiple thermal recording layers formed on a support in which at least one thermal recording layer of the present invention. In the full-color thermal recording material, it is desirable that at least one thermal recording layer is a photo-fixing recording layer that contains a diazonium salt compound and a coupler capable of reacting with the diazonium salt compound to form a color.
particularly when the full-color thermal recording material has cyan, yellow and magenta-forming thermal recording layers, it is desirable that the first thermal recording layer nearest to the support contains a leuco-type color former of an electron-donating dye precursor and an electron-accepting compound, and the second and third thermal recording layers contain a diazo-type color former.
For example, the thermal recording material may be formed by embodiments expressed by the following (a)-(c).
(a) A recording material comprising a laminated structure of; a photo-fixing recording layer that contains the low sensitivity microcapsules of the present invention which encapsulate a diazonium salt compound (maximum absorption wavelength of 365xc2x140 nm) therein, and a coupler capable of reacting with the diazonium salt compound to form a dye (first recording layer A); and a photo-fixing recording layer that contains a diazonium salt compound (maximum absorption wavelength of 420xc2x140 nm) and a coupler capable of reacting with the diazonium salt compound to form a dye (second recording layer B). This recording material may have a light transmittance-controlling layer and a protective layer on the laminate structure, according to necessity.
(b) A recording material comprising a laminated structure of; a recording layer that contains the low sensitivity microcapsules of the present invention, which encapsulate an electron-donating dye precursor, and an electron-accepting compound (first recording layer A); a photo-fixing recording layer that contains a diazonium salt compound (maximum absorption wavelength of 365xc2x140 nm), and a coupler capable of reacting with the diazonium salt compound to form a dye (second recording layer B); and a photo-fixing recording layer that contains a diazonium salt compound (maximum absorption wavelength of 420xc2x140 nm) and a coupler capable of reacting with the diazonium salt compound to form a dye (third recording layer C) laminated on the support in that order. This recording material may have a light transmittance-controlling layer and a protective layer on the laminate structure, according to necessity.
(c) A recording material comprising a laminated structure of; a photo-fixing recording layer that contains the low sensitivity microcapsules of the invention, which encapsulate a diazonium salt compound (maximum absorption wavelength of at most 350 nm), and a coupler capable of reacting with the diazonium salt compound to form a dye (first recording layer A); a photo-fixing recording layer that contains a diazonium salt compound (maximum absorption wavelength of 365xc2x140 nm), and a coupler capable of reacting with the diazonium salt compound to form a dye (second recording layer B); and a photo-fixing recording layer that contains a diazonium salt compound (maximum absorption wavelength of 420xc2x140 nm) and a coupler capable of reacting with the diazonium salt compound to form a dye (third recording layer C) laminated on a support in that order. The recording material may have a light transmittance-controlling layer and a protective layer on the laminate structure, according to necessity.
When a surfactant such as that mentioned hereinabove is used in forming the low sensitivity microcapsules of the present invention to be in the laminate layer structure of the thermal recording material, the microcapsules can be formed stably. Therefore, in the thermal recording material of the present invention, it is preferable that the thermal recording layer with the microcapsules therein be further coated with at least one other thermal recording layer on its recoding surface, and it is more preferable that the thermal recording layer with the microcapsules therein is nearest to the support (lowermost layer).
Methods of forming full-color images on the embodiments (b) and (c) are described below.
First, the third recording layer (C) is heated, and the diazonium salt is reacted with the coupler therein to form a dye. Next, the recording material is exposed to light of which the emission center wavelength is 430xc2x130 nm, whereby the non-reacted diazonium salt compound in the layer C is decomposed and the image, which has been formed is photo-fixed. Then, the recording material is exposed to heat sufficient to form a dye in the second recording layer (B), and the diazonium salt compound in the layer (B) is reacted with the coupler therein to form a dye. In this stage, the layer C is also strongly heated, but the diazonium salt compound therein has been already decomposed (that is, the image formed has been photo-fixed), and has lost its color-forming ability. Next, the recording material is exposed to light of which the emission center wavelength is 360xc2x120 nm, whereby the diazonium salt compound in the layer B is decomposed and the image formed is photo-fixed. Finally, the recording material is exposed to heat sufficient to form a dye in the first recoding layer (A). In this stage, the layers (B) and (C) are also strongly heated, but the diazonium salt compounds therein have been already decomposed and have lost their color-forming ability. In cases where the thermal recording layer of the invention is the layer (A), it is preferable that the sensitivity of the microcapsules therein be no more than 60 mJ/mm2 in terms of the recording energy to form yellow of O.D.=1.5, and more preferably no more than 55 mJ/mm2. In cases where the thermal recording layer of the invention is the layer (B), it is preferable that the sensitivity of the microcapsules therein be no more than 100 mJ/mm2 in terms of the recording energy to form magenta of O.D.=1.5, and more preferably no more than 95 mJ/mm2. Preferably, the sensitivity to heat of the microcapsules with an electron-donating dye precursor or a diazonium salt compound encapsulated therein in the thermal recording material of the invention is low such that the color density of the image formed, in terms of the optical density, O.D. at a recording energy of 100 mJ/mm2, is no more than 0.2. More preferably, the sensitivity to heat of the microcapsules with an electron-donating dye precursor or a diazonium salt compound encapsulated therein in the lowermost layer (A) in the thermal recording material of the invention is low such that the color density of the image (cyan image) formed, in terms of the optical density, O.D. at recording energy of 100 mJ/mm2, is no more than 0.2. Controlling the sensitivity to heat of the microcapsules of the invention in the layer (A) (lowermost layer) to the range as above makes it possible to prevent the layer (A) from forming a color when heat is applied to the layer (B). Accordingly, color mixing in the image formed on the recoding material of the invention is prevented, and the thermal differentiation in the recording material is improved.
In cases where all the recording layers (A, B and C) in the recording material are diazo-type recording layers, the layers (A) and (B) must be photo-fixed after processed for color image formation, but the layer (C), which is finally processed for color image formation last, does not necessarily require photo-fixing.
The light source for photo-fixation may be selected from any known light sources. Examples include various fluorescent lamps, xenon lamps and mercury lamps. A light source of having an emission spectrum is almost the same as the absorption spectrum of the diazonium salt compound used in the recording material is especially preferable, as it enables high-efficiency photo-fixation.
Other Layers:
The thermal recording material of the present invention that has one or more thermal recording layers on a support preferably also has a light transmittance-controlling layer and a protective layer.
Light Transmittance-Controlling Layer:
The light transmittance-controlling layer contains a UV absorbent precursor, in which the precursor does not function as a UV absorbent before the layer is exposed to light necessary for image fixation. Therefore, before being exposed to light for image fixation, the light transmittance of the layer is high. When the photo-fixing thermal recording layers are exposed to light for image fixation thereon, the light transmittance-controlling layer transmits the light for image fixation well and transmits visible light well, and therefore does not interfere with image fixation on the recording layers. The UV absorbent precursor in the light transmittance-controlling layer is preferably encapsulated into microcapsules.
Examples of the compounds that may be in the light transmittance-controlling layer are given in JP-A No. 9-1928.
After the thermal recoding layers have been exposed to light for image fixation thereon, the UV absorbent precursor in the light transmittance-controlling layer receives light or heat and becomes able to funstion as a UV absorbent. Accordingly, almost all light falling within a UV range is absorbed by the UV absorbent in the light transmittance-controlling layer, and the UV light transmittance of the layer is lowered. As a result, the lightfastness of the image-recorded material is improved, but since the UV absorbent does not absorb visible light, the visible light transmittance of the image-recorded material does not substantially change.
At least one light transmittance-controlling layer may be in the thermal recording material of the invention, and most preferably, between the thermal recording layer and the outermost protective layer. However, the light transmittance-controlling layer may also serve as a protective layer. The characteristics of the light transmittance-controlling layer may be suitably determined, depending on the characteristics of the thermal recording layers in the recording material.
A coating liquid for forming the light transmittance-controlling layer (coating liquid for light transmittance-controlling layer) may be prepared by mixing the components mentioned above. The coating liquid can be applied onto the recording layer-coated material by any known coating method using, for example, a bar coater, an air knife coater, a blade coater or a curtain coater. The light transmittance-controlling layer may be formed simultaneously with the thermal recording layers; or after the coating liquids for the thermal recording layers have been applied onto a support and been dried thereon, the coating liquid for the light transmittance-controlling layer may be applied onto the recording layers.
A dry weight of the light transmittance-controllinglayer formed is preferably from 0.8 to 4.0 g/m2.
Protective Layer:
The protective layer contains a binder, and also a pigment, a lubricant, a surfactant, a dispersant, a fluorescent brightener, a metal soap, a hardener, a UV absorbent, a crosslinking agent, and the like.
The binder may be any, which has not inhibit the barrier property of the protective layer or the workbility of the layer. Examples include polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, starches, gelatin, arabic gum, casein, styrene-maleic anhydride copolymer hydrolyzate, ethylene-maleic anhydride copolymer hydrolyzate, isobutylene-maleic anhydride copolymer hydrolyzate, polyvinyl alcohol, modified polyvinyl alcohol, and polyacrylamide.
In addition to the above, synthetic rubber latexes and synthetic resin emulsions are also usable as the binder. For example, styrene-butadiene rubber latex, acrylonitrile-butadiene rubber latex, methyl acrylate-butadiene rubber latex, and polyvinyl acetate emulsion.
The binder content of the protective layer is preferably from 10 to 500% by weight, more preferably from 50 to 400% by weight of the pigment in the layer.
For improving the waterproofness of the protective layer, a crosslinking agent combined with a catalyst for promoting its reaction is effective. Examples of the crosslinking agent include epoxy compounds, blocked isocyanates, vinylsulfone compounds, aldehyde compounds, methylol compounds, boric acids, carboxylic acid anhydrides, silane compounds, chelate compounds and halide compounds, and those capable of controlling the pH of the coating liquid for the protective layer from 6.0 to 7.5 are preferable. The catalyst may be any known acid or metal salt. For it, those capable of controlling the pH of the coating liquid to be from 6.0 to 7.5 are also preferred.
Any known organic or inorganic pigment may be used in the protective layer. Specifically, examples of the pigment include calciumcarbonate, aluminium hydroxide, barium sulfate, titanium oxide, talc, agalmatolite, kaolin, calcined kaolin, amorphous silica, colloidal silica, urea-formalin resin powder, polyethylene resin powder, and benzoguanamine resin powder. One or more of these may be in the layer singly or in combinations of two or more.
Preferable examples of the lubricant that may be in the protective layer are zinc stearate, calcium stearate, paraffin wax, and polyethylene wax.
A surfactant is added to the coating liquid for the protective layer so that the protective layer can be uniformly formed on the thermal recording layers. Preferable examples thereof are alkali metal sulfosuccinates and fluorine-containing surfactants, and specifically, they are sodium salts and ammonium salts of di(2-ethylhexyl)sulfosuccinic acid and di(n-hexyl)sulfosuccinic acid.
The coating liquid for forming the protective layer (coating liquid for protective layer) may be obtained by mixing the above-mentioned components. If desired, a release agent, wax and a water repellent may be added thereto.
To produce the thermal recording material of the invention, the coating liquid may be applied to the thermal recording layers formed on a support to thereby form the protective layer thereon, by any ordinary coating method. For example, a bar coater, an air knife coater, a blade coater or a curtain coater may be used.
The protective layer may be formed simultaneously with the thermal recording layers and the light transmittance-controlling layer. Alternatively, the protective layer may be formed on the recording layers after the coating liquids for the thermal recording layers are applied onto a support and dried.
The dry weight of the protective layer is preferably from 0.2 to 7 g/m2, more preferably from 1 to 4 g/m2. If the dry weight is lower than 0.2 g/m2, the protective layer might not be able to maintain water resistance; but if higher than 7 g/m2, the thermal sensitivity of the recording material might be significantly lowered. After the protective layer has been formed, it may be optionally calendered.
Interlayer:
When the thermal recording layer comprises multiple layers, providing an interlayer between the neighboring thermal recording layers is preferable. Like the protective layer, the interlayer may contain a binder and also a pigment, a lubricant, a surfactant, a dispersant, a fluorescent brightener, a metal soap, a UV absorbent, and the like. The binder in the interlayer may be the same as that in the protective layer.
Support:
The support for the thermal recording layer of the invention is made of, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), triacetyl cellulose (TAC), paper, plastic resin-laminated paper, or synthetic paper. For transparent thermal recording materials, the support must be transparent. The transparent support may be a synthetic polymer film, including, for example, polyester films of polyethylene terephthalate or polybutylene terephthalate, cellulose triacetate film, and polyolefin films of polypropylene or polyethylene.
The support maybe a single-layered support or a laminated support.
The thickness of the synthetic polymer films for the support is preferably from 25 to 300 xcexcm, and more preferably from 100 to 250 xcexcm.
The synthetic polymer films may be colored in any desired manner. Examples of coloring methods include  less than 1 greater than  previously kneading a dye in resin before the resin is formed into films, and the dye-containing resin is formed into a film; or  less than 2 greater than  dissolving a dye in a suitable solvent to prepare a dye solution, and applying this onto a transparent colorless resin films in an ordinary coating method, and dried thereon. Examples of the coating and drying methods include gravure coating, roller coating and wire coating. Among these examples, films of polyethylene terephthalate or polyethylene naphthalate containing a blueing dye are preferable. The films may be thermally processed, stretched or processed for static charge prevention.
The thermal recording layers, the protective layer, the light transmittance-controlling layer and the interlayer mentioned above may be formed on a support by any known coating method, and dried thereon. Examples of the known coating method include blade coating, air knife coating, gravure coating, roll coating, spraying, dipping, and bar coating. If desired, the back surface of the support, which is not coated with the thermal recording layers may be coated with a back coat layer, or an adhesive layer (sealant layer) combined with a release layer.